LOS ANGELES — The arrest in the case of the "Grim Sleeper" — a serial killer who terrorized South Los Angeles for two decades — has put one of the hottest controversies in American law enforcement to its first major test.

Only two states, Colorado and California, have a codified policy permitting a so-called familial search, the use of DNA samples taken from convicted criminals to track down relatives who may themselves have committed a crime. It is a practice that district attorneys and the police say is an essential tool in catching otherwise elusive criminals, but that privacy experts criticize as a threat to civil liberties.

This week, law enforcement struck a significant blow for the practice when the Los Angeles Police Department used it to arrest a man who they say murdered at least 10 residents here over 25 years. It is the first time an active familial search has been used to solve a homicide case in the United States.